clock_control: stm32f1: fix STM32_PLL_XTPRE check
clock_control: stm32f1: fix STM32_PLL_XTPRE check

This is always defined since:

755d09e149 include/drivers/clock_control: stm32: Update for STM32F1
support

So the condition has currently no effect and causes the prescaler to
always be set to /2.

Signed-off-by: Fabio Baltieri <161d407f4dc681bfd4aedb33b8273b2aafd727bc@gmail.com>

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